(1) Field of the Invention
The present invention generally relates to an exhaust emission control apparatus, and more particularly to an exhaust emission control apparatus in which an absorbent for absorbing hydrocarbon and a catalytic converter for purifying exhaust gas are provided in an exhaust passage.
(2) Description of the Related Art
In order to purify exhaust gas in an exhaust passage of an internal combustion engine of an automobile vehicle, a pellet catalyst or a monolithic catalyst is conventionally used in an exhaust system of the engine. Hydrocarbon HC is one of the harmful components (HC, CO, NOx) in the exhaust gas, and purification of hydrocarbon HC performed by means of a catalytic converter is strongly dependent on the exhaust gas temperature. Even when a precious metal catalyst is used to purify the exhaust gas, the exhaust gas temperature must be higher than 250 deg C to efficiently purify hydrocarbon HC in the exhaust gas.
Therefore, it is difficult to purify hydrocarbon HC in exhaust gas by means of a catalytic converter especially when the exhaust gas is at a relatively low temperature just after the engine has started to operate. Moreover, just after the engine has started to operate, a large amount of hydrocarbon HC is emitted from the engine (which is hereinafter called cold HC), and the amount of this cold HC relative to a total of the exhaust gas is large. Thus, it is necessary to control the emission of cold HC in exhaust gas when the exhaust gas is at a low temperature just after the engine has started to operate, in order to improve the performance of the engine.
To eliminate the above described problem, a certain type of exhaust emission control apparatus has been proposed. This apparatus absorbs hydrocarbon HC in exhaust gas using a HC absorbent trapper when the exhaust gas temperature is low, whereas, when the exhaust gas temperature is high, the harmful components of exhaust gas as well as the cold HC desorbed from the HC absorbent are purified by means of a catalytic converter. Japanese Laid-Open Patent Publication No. 2-173312 discloses a typical exhaust emission control device of the type mentioned above. In the exhaust emission control device disclosed in the above mentioned publication, an HC absorbent trapper for absorbing the cold HC and a catalytic converter for purifying the exhaust gas are provided in the exhaust system of the engine.
However, in the exhaust emission control device mentioned above, the HC absorbent trapper is placed in a by-pass passage, and the catalytic converter is placed in an intermediate portion of an exhaust passage. The by-pass passage is arranged so as to branch from the exhaust passage at an upstream portion of the converter and it is connected to the exhaust passage at a downstream portion of the converter. A switching valve is arranged within the exhaust passage in the branch portion of the by-pass passage, so that exhaust gas from the engine can be fed into either the exhaust passage or the by-pass passage. That is, the switching valve is switched on and off in a manner such that when the exhaust gas is at a relatively low temperature (below a catalyst activation temperature) the exhaust gas is fed into the by-pass passage in which the HC absorbent trapper is placed, whereas, when the exhaust gas is at a high temperature (above the catalyst activation temperature), the exhaust gas is fed into the exhaust passage via the switching valve.
However, in the above described conventional device, no consideration is given to the fact that the HC absorbent and the catalytic converter have different temperature characteristics. Generally speaking, at a temperature lower than the desorption temperature (which lies in a range between 100 deg C. and 200 deg C.), the HC absorbent trapper efficiently absorbs hydrocarbon HC in exhaust gas, whereas the absorbed HC is desorbed from the HC absorbent at a temperature higher than the desorption temperature. The activation temperature of a catalytic converter (at which the catalyst of the catalytic converter is activated) is around 250 deg C. when in an initial condition, and the activation temperature thereof changes to a higher temperature after the catalytic converter has been used for a long period of time. The HC absorbent temperature and the catalytic converter temperature in the above mentioned exhaust emission control device increase at the same time due to the heat of the exhaust gas. Thus, the rate of increase of the temperature of the HC absorbent at the upstream portion of the exhaust passage relative to the flow of the exhaust gas is greater than the rate of increase of the temperature of the catalytic converter at the downstream portion thereof.
As described above, in the above conventional device, no consideration is given to the difference between the temperature characteristics of the HC absorbent and the catalytic converter. More specifically, even when the HC absorbent temperature is higher than the desorption temperature (around 200 deg C.) and the absorbed HC is gradually desorbed from the HC absorbent, the catalytic converter temperature is still lower than the activation temperature (around 250 deg C.). The hydrocarbon desorbed from the HC absorbent cannot be purified by the inactive catalyst of the catalytic converter at such a temperature, and thus there is a problem in that the desorbed hydrocarbon is emitted to the atmosphere in an unpurified state.
Moreover, the exhaust passage in which the exhaust emission control device is provided is often subjected to vibrations due to the operating of the engine and the vehicle running on a rough road. Thus, the above conventional exhaust emission control device is strongly influenced by such subjecting of the exhaust passage to vibrations as well as by the exhaust back pressure changes, and thus it is difficult to correctly control the operations of the switching valve for selectively feeding the exhaust gas into the exhaust passage or the by-pass passage. In addition, there is a problem in that a certain amount of hydrocarbon HC may leak from around the switching valve, and thus the HC is emitted to the atmosphere in an unpurified state. Also, there is a problem in that use of the by-pass passage makes the conventional device somewhat bulky and complicated.